Made the image_dataset_metadata::box constructable from a rectangle.

dlib/data_io/image_dataset_metadata.h

@@ -27,6 +27,7 @@ namespace dlib
                     The main variable of interest is rect.  It gives the location of 
                     the box.  All the other variables are optional.
             !*/
+
             box(
             ) : 
                 difficult(false),
@@ -36,6 +37,17 @@ namespace dlib
                 angle(0)
             {}
 
+            box (
+                const rectangle& rect_
+            ) : 
+                rect(rect_), 
+                difficult(false),
+                truncated(false),
+                occluded(false),
+                ignore(false),
+                angle(0)
+            {}
+
             rectangle rect;
 
             std::map<std::string,point> parts;